1. Field of the Invention (Technical Field)
The present invention relates to a system and method for controlling a burner's adjustable secondary air controller. Specifically, the system relates to a control logic for the control of an air supply to a burner such as a natural gas burner used in the production and/or processing of liquid and gaseous petroleum products and in natural gas dehydration systems.
2. Background Art
Burners are used for various applications such as, for example, the production and/or processing of liquid and gaseous petroleum products. However, the typical burner with no variable air controller utilizes an excessive amount of air and oxygen, and such use is inefficient and wasteful since that excessive, unnecessary air is heated.
To increase efficiency, the variable air controller described in U.S. Pat. No. 4,568,268, was directed to controlling the total air flowing into the burner housing as a function of the pressure of the natural gas supplied to the burner. That air controller regulates the amount of air, and thus the desired 3% to 5% excess oxygen, and achieves a significant savings of fuel gas. In addition to allowing a primary supply of air which is always available, the secondary air controller controls the overall excess oxygen by controlling only a secondary air flow. Therefore, that air controller allows regulation of the air fuel ratio over the firing range for 3% to 5% of excess air.
However, the air controller of U.S. Pat. No. 4,568,268 is prone to several problems. When the burner is at a low firing condition or completely off, many times low stack temperatures, as a result of reducing the air flow in the stack, resulted in stack sweating. Stack sweating was, at times, severe enough to cause water to flow back through the firetube and out of the breech of the flame arrestor. When the burner is off and the secondary air controller is closed, there is insufficient oxygen in the firetube to support firing of the main burners. When the process calls for heat, the main burners will begin firing before the secondary air controller can open creating a lack of oxygen and minor explosions in the firetube. The minor explosions are commonly referred to as “wolfing”.
Also, the use of throttling thermostats, which are preferable in many applications, can create firing problems when the heat requirements for the process are low and the gas pressure to the burner is below the pressure required to create the gas velocity needed to properly draw and mix the primary air in the burner venture.
It is also difficult to adjust that air controller. Adjustment of the secondary air controller involves a trial and error changing of the compression springs which oppose the movement of the diaphragm on the secondary air controller. Differences in firetube size, altitude, gas composition, etc. requires adjustment of the secondary air controller. The adjustment entails firing the burner and analyzing the stack gases. If the stack gases do not meet the requirements, the burners are shut down, the compression springs are changed, the burner is relit, and the stack gases are again analyzed. The process of firing analyzing and changing compression springs is continued until the right composition of stack gases is obtained. Getting the correct compressions springs installed has proven to be, at times, a long and difficult task.